1. Technical Field of the Invention
The present invention relates to piezoelectric resonators and electronic components including such piezoelectric resonators, and more particularly, to a piezoelectric resonator which maximizes the use of the mechanical resonance of a piezoelectric member, and electronic components including such a piezoelectric resonator, such as an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 37 is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a piezoelectric substrate 2 having, for example, a rectangular plate shape as viewed from above. The piezoelectric substrate 2 is polarized in the thickness direction thereof. Electrodes 3 are provided on both major surfaces of the piezoelectric substrate 2. When a signal is input between the electrodes 3, an electrical field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in the longitudinal direction. In FIG. 38, there is shown a piezoelectric resonator 1 in which electrodes 3 are disposed on both major surfaces of a piezoelectric substrate 2 having a square plate shape as viewed from above. The piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 of the piezoelectric resonator 1, an electrical field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in a square-type vibration mode.
These piezoelectric resonators are of an unstiffened type, in which the vibration direction differs from the direction of polarization and the direction of application of electrical field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. This causes a problem in that a frequency bandwidth in use is narrow when an unstiffened frequency resonator is used as an oscillator or a filter. Therefore, the degree of freedom in characteristics design is low in such a piezoelectric resonator and electronic components including such a piezoelectric resonator.
The piezoelectric resonator shown in FIG. 37 generates a first-order resonance in the longitudinal mode. It also generates, due to its structure, large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and spurious resonances in a width mode. To suppress these spurious resonances, some corrective measures to be applied to the piezoelectric resonator have been considered, such as polishing the resonator surfaces, increasing mass, and changing the shape of the electrodes. These corrective measures increase manufacturing cost, time and difficulty.
Because the piezoelectric substrate of the piezoelectric resonator shown in FIG. 37 has a rectangular plate shape, the substrate cannot be made thinner because strength requirements necessitate that the substrate have a minimum thickness. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be increased. These disadvantages and limitations on the design and manufacture of the resonator makes it extremely difficult to achieve impedance matching with an external circuit.
To form a ladder filter by connecting a plurality of piezoelectric resonators in series and in parallel alternately, the capacitance ratio of the series resonator to the parallel resonator must be large in order to increase attenuation in zones other than the pass band. Because a piezoelectric resonator has the shape and configuration restrictions described above, however, a large attenuation may not be obtained.
The piezoelectric resonator shown in FIG. 38 generates first-order resonance in the square-type mode. Because of its structure, large spurious resonances such as those in the thickness mode and in the third-harmonic mode in the plane direction are very likely to be generated. Since the piezoelectric resonator must have a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of piezoelectric resonator of the type shown in FIG. 38.
When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series have increased thicknesses and electrodes are formed only on part of a piezoelectric substrate to make the capacitance small. Since the electrodes are only partially formed on the surfaces of the piezoelectric substrate, the difference .DELTA.F between the resonant frequency and the antiresonant frequency and the capacitance is reduced. The resonators connected in parallel are therefore required to have small .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission bandwidth of the filter cannot be increased.
A piezoelectric resonator having a small spurious resonance and a large difference .DELTA.F between the resonant frequency and the antiresonant frequency has been considered in Japanese patent application number 8-110475. FIG. 39 is a view of a piezoelectric resonator having such a structure. In the piezoelectric resonator 4 shown in FIG. 39, a plurality of piezoelectric layers 6 and a plurality of electrodes 7 are alternately laminated to form a narrow base member 5, and the plurality of piezoelectric layers 6 are polarized in the longitudinal direction of the base member. This laminated piezoelectric resonator 4 is a stiffened type, and the piezoelectric layers 6 are arranged such that the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electric field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. In addition, vibrations in modes such as the width and thickness modes, which are different from the basic vibration, are unlikely to occur in the piezoelectric resonator 4 having the lamination structure described above and shown in FIG. 39.
In the piezoelectric resonator 4 having this lamination structure, the ends of the electrodes 7 are exposed at all side surfaces of the base member 5. Therefore, on a first side surface of the base member 5, the ends of alternate electrodes 7 are covered by insulating resin films 8a and an external electrode 9a is provided so as to be connected to the other alternate electrodes 7. On a second side surface of the base member 5, which is opposite to the first side surface, the ends of the second alternate electrodes 7 are covered by insulating resin films 8b and an external electrode 9b is arranged so as to be connected to the first alternate electrodes 7, on which the insulating resin films 8a are provided. The capacitance C between the external electrodes 9a and 9b is expressed by: C.varies.nS/T in the piezoelectric resonator 4 having this lamination structure, where S indicates the area of a cross section which is perpendicular to the longitudinal direction of the base member 5 or the area of the main surface of a piezoelectric layer 6, T indicates the thickness of a dielectric layer 6 or the distance between electrodes 7, and "n" indicates the number of layers between electrodes 7. Therefore, in the piezoelectric resonator 4 having this lamination structure, to obtain the same capacitance if the area S is decreased to reduce the size of the resonator, it is necessary to reduce T or increase "n." Since it is difficult to form the insulating resin films 8a and 8b precisely at a desired position in a compact piezoelectric resonator having a distance of, for example, 100 .mu.m or less between electrodes 7 by a method such as printing, it is difficult to make a piezoelectric resonator having a substantially reduced and compact size and configuration.
Also in the piezoelectric resonator 4 having such a lamination structure, since the external electrodes 9a and 9b provided on the insulating resin films 8a and 8b have a different thermal expansion coefficient from that of the insulating resin films 8a and 8b, the external electrodes 9a, 9b may break, be fractured or damaged and possibly be partially or completely separated from the insulation resin films 8a and 8b as shown in FIG. 39 due to thermal shock or thermal cycling in a subsequent processing step.